1. Field of the Invention
The present invention relates generally to automatic focusing apparatuses, and more specifically, to an automatic focusing apparatus for automatically matching the focus to an object in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera having an automatic focusing function.
2. Description of the Background Art
Conventionally, in an automatic focusing apparatus for use in an image sensing apparatus such as a video camera, an approach utilizing a video signal obtained from an image sensor itself for evaluation for the state of focus control has been developed. Such an approach is essentially free from parallax and possesses excellent characteristics that, for example, the focus can exactly be matched even if the depth of field is small and an object is located at a distance. Furthermore, according to this approach, a special sensor for automatic focusing does not have to be separately provided and therefore the apparatus is of a very simple mechanism.
As an example of such a focus control method utilizing a video signal, a so-called hill-climbing servo system has been conventionally known. An automatic focusing apparatus utilizing the hill-climbing servo system is disclosed, for example, in U.S. Pat. Nos. 4,922,346 and 5,003,339, and briefly stated, a high frequency component of a video signal from an image sensor is detected for every field as a focus evaluating value, the detected focus evaluating value is always compared with a focus evaluating value detected one field before, and a focus lens continues to be slightly vibrated so that the focus evaluating value always takes a maximum value.
In recent years, as digitization of signal processing circuitry, for example, in digital video tape recorders has developed, there are demands for digital automatic focusing circuitry.
For example, replacement of an analog high-pass filter (hereinafter referred to as HPF) in an automatic focusing circuit with a digital HPF is demanded. An infinite impulse response (IIR) filter would be suited for the digital HPF in such a case for its characteristic and in view of its circuit scale.
As illustrated in a timing chart in FIG. 10, for example, a video signal originally does not have a high frequency component when the picture plane is entirely white. Therefore, a digital HPF will output nothing in response to such a video signal.
However, the horizontal period (field period) of the video signal includes a horizontal blanking period for a horizontal retrace line, and the video signal has rising and falling edges before and after the horizontal blanking period. Accordingly, high frequency components for these edges are output from the digital HPF. Such a phenomenon is called ringing.
Especially when the digital HPF includes an IIR filter, an output signal from the HPF attenuates while ringing, but if the time constant of the HPF is large, integration is initiated before an output signal from the HPF is sufficiently attenuated. Thus, part of ringing is mixed into the focus evaluating value as a pseudo signal and adversely affects the focusing operation.
Meanwhile, when a signal processing circuit is digitized, power consumption is reduced. In that regard, a clock signal is supplied for operating the signal processing circuit only when it is necessary, and stopping the clock signal otherwise may be usually employed.
Therefore, an output signal from the HPF is not necessary other than in the period for integration in the automatic focusing circuit, and therefore as illustrated in FIG. 11, it is considered to provide a period for stopping the clock signal.
During the period in which the clock signal is stopped, the value of a delay register in the digital HPF does not change, and the delay register maintains the video signal level at a level that occurred immediately before the onset of the period during which the clock stopped period. Accordingly, if there is a difference between the level of a video signal immediately before the clock stopped period and the level of the video signal immediately after the clock stopped period, an edge V is substantially present at the end of the clock stopped period.
Under such a situation, a high frequency component due to edge V is output from the HPF similarly to the above-described case. If integration is initiated before the output signal from the HPF is sufficiently attenuated, part of the ringing is mixed into the focus evaluating value as a pseudo signal, similar to the above case, adversely affects the focusing operation.